One of the things that I strongly suggest of students in the drawing class is that they email me work in progress. Especially if they run into a problem with a homework assignment or critique piece - it's easy to just snap a picture with a smartphone and email them for input. Here's a sample of some suggestions for some thumbnail roughs done in preparation of the first linear perspective piece, and arrangement of objects on a desktop shown in correct perspective, ie looking like the items are actually on the surface plane. Using quick & dirty modifications in Photoshop I can show how minor tweaks such as shifting the elements and/or cropping and zooming in on the piece will lead to a better, more tighter composition.
Under ideal conditions, and with some more experience, this sort of editing all takes place between the eyes, brain, pencil and subject of the sketch. Reality rarely gives a one-shot opportunity to faithfully copy a good drawing from life (besides which, spare yourself the frustration and time by taking a digital picture, which in theory ought to have freed the artist from slavishly emulating reality): one must almost always, to some extent, make stuff up.
I also employ Photoshop when taking a sample piece and showing how even a horrid quality picture taken with an iPhone under lousy lighting can be tweaked for notable improvement and serve as a suggestion as to what and where one can rework the original piece. And again, to some degree these compositional challenges should ideally be addressed first in the sketchbook with roughs before putting anything down for the critique piece.